Wound rotors having laminated stacks are well known. The stacks are formed from a number of laminated steel members each having a plurality of "T"-shaped arms extending radially from a central hub. The hub has a central hole for mounting the laminated steel member on a shaft. The distal end of the arms have circumferentially extending projections to form the familiar "T"-shape with the projections facing adjacent projections across a small gap. The area defined by adjacent arms is known as a winding tunnel and when the rotor is being wound, the wire of the windings (also known as magnet wire) is passed through the small gaps so as to be located in the winding tunnels.
For many, motors, such as A.C. motors operating at line or mains voltage, the winding tunnels are lined with insulation, called slot insulation, to isolate the windings from the rotor stack. At the end of the stack, it is known to provide an armature end protector, also known as a spider because of its appearance, to maintain the separation between the magnet wire and the rotor stack. It is common for the slot insulation to be a sheet of insulation material such as MYLAR paper which is located in and line the walls of the tunnels. These sheets do not wrap around the ends of the winding tunnels and thus, the spiders are needed to guarantee the creepage distance or separation between the rotor stack and the windings.
It is known to make these spiders from fiber material, thermosetting plastic material and from thermosoftening plastic material. It is usual to provide spacers between the spiders and the commutator and fan. The spacers not only regulate the distance between the rotor stack and the commutator and fan but also provide an insulated sleeve over the shaft where the wire would otherwise lay on the shaft. The spacers tires need to be made of insulating material which can withstand the heat generated by the windings and be sufficiently thick to maintain the required separation.
Gaps between the spacers and spiders may occur during faulty assembly or during use which may lead to insulation failure. A spider and spacer combination overcome this problem and thus are preferred. However, spacers made from fiber material are too expensive for normal applications.
Single piece combined spiders and spacers made from plastic materials work satisfactorily but they do have disadvantages. Thermosoftening plastic generally cannot withstand the heat which can be developed in modern motors under abnormal conditions such as when operating under stall conditions, leading to melting and distortion of the spider portion and/or spacer portion Which may result in a breach of the separation requirement leading to insulation failure.
Spiders of thermosetting plastic material, on the other hand, can tolerate the high temperature of the windings but tend to be brittle. For smaller motors, the diameter of the rotor and the spider is small. The spokes of the spider are thin and, due to their brittle nature, break easily. Hence, for small multislot motors, the use of thermosetting plastic material spiders is not viable due to the care required during assembly and handling to avoid breaking the spokes of the spider.
Epoxy coating of the rotor stack is also known as an alternative to using slot insulation and spiders. However, as it is difficult to ensure that the stack is evenly coated with a sufficient depth of epoxy, especially on the edges and without pinholes, its application is generally restricted to low voltage applications.